It is also important to bear in mind that Japan's strategic aims were not set in concrete in 1942. Those aims changed as the tide of battle ebbed and flowed against Japan or in its favour. Important changes in Japan's strategic aims were produced during 1942 by events such as the carrier-launched Doolittle Raid on Japan and as the tide of the Pacific War turned against Japan. Those changes are mentioned elsewhere on this web-site in the context of the Battle of the Coral Sea , the Battle of Midway , the Kokoda Campaign , and the Guadalcanal Campaign .
When a new edition was released, the cryptographers were forced to start again. The original JN-25A system replaced the 'Blue' code (as Americans called it), and used five-digit numbers, each divisible by three (and so usable as a quick, and somewhat reliable, error check, as well as something of a 'crib' to cryptanalysts), giving a total of 33,334 legal code values. To make it harder to crack a code value, meaningless additives (from a large table or book of five-digit numbers) were added arithmetically to each five-digit cipher element. JN-25B superseded the first release of JN-25 at the start of December 1940. JN-25B had 55,000 valid words, and while it initially used the same additive list, this was soon changed and the cryptanalysts found themselves entirely locked out again.